abc's jim avila has the breaking news. >> hazmat suited inspectors gathered evidence linked to a letter addressed to the president of the united states, another to a mississippi senator, federal sources tell abc news they've arrested a suspect from tupelo, mississippi. a scare that stopped mail delivery on capitol hill this week. >> all staff and other personnel are directed to avoid this area until further notice. >> reporter: the fear, ricin. easily made from castor beans, by would-be bioterrorists. >> if you ingest enough, it could be very damaging and it could kill you. >> reporter: two letters, one to president obama, another two roger wicker, showed initial positive results in field tests. the letters never reached the white house or capital. >> the mail sent here is screened and the tests are taken at remote sites to mitigate the risks. >> the first was stopped then miles from its target to capitol hill. the second letter to the white house was intercepted six and a half miles away on a military base on the outskirts of d.c. a third post office sorting facility suffered possible